This invention relates to the preparation of polyurethane foarms. This invention further relates to the use of certain latent organotin catalysts to prepare urethane foams that exhibit a high level of oxidative stability, particularly at elevated temperatures, relative to products obtained using other conventional organotin catalysts.
Flexible polyurethane foams are widely used in the manufacture of furniture, particularly seat cushions, and as packaging material for delicate instruments and other articles that are susceptible to damage during handling and transit. If the foam is incorporated into a piece of furniture or other durable product, the foam must withstand exposure of several years or longer to elevated temperatures, atmospheric oxygen, or both without undergoing significant degradation as evidenced by a gradual loss of structural strength, which may culminate in disintegration of the foam.
It is well known to prepare cellular polyurethanes by reacting polyols containing two or more reactive hydrogen atoms, as determined by the Zerewitinoff method, with polyfunctional isocyanates in the presence of a polymerization or gel catalyst and a blowing agent such as water or a relatively low boiling fluorinated hydrocarbon. A surfactant is often included in the reaction mixture together with a blowing catalyst to obtain the desired small, uniform cell size within the foam.
U.S. Pat. No. 3,620,985 discloses that both divalent and tetravalent tin compounds are effective gel catalysts for cellular polyurethanes. The divalent tin compounds, exemplified by stannous salts of carboxylic acids such as stannous octoate, are so susceptible to oxidation that they decompose relatively rapidly in the presence of air. Special handling and storage of these stannous compounds are therefore required to retain their activity as catalysts. Compounds of the formula R.sub.a SnX.sub.4-a wherein R is typically butyl or other alkyl radical containing from 1 to 20 carbon atoms and X is halogen or other anionic radical are less than satisfactory for preparing flexible foams that are exposed to atmospheric oxygen, elevated temperatures, i.e. above about 50.degree. C. or both over extended periods of time. Under these conditions the foams may lose resiliency and structural integrity, sometimes to the extent that they disintegrate when compressed. This is particularly true for those foams derived from polyols that are reaction products of propylene oxide and glycerine. These polyols may also contain end groups derived from ethylene oxide to obtain hydroxyl groups bonded to primary rather than secondary carbon atoms. Polyurethanes derived from polyols that contain side chains resulting from the graft polymerization of acrylonitrile, styrene or other vinyl monomer onto a poly(propylene oxide) backbone are usually less susceptible to oxidative and thermal degradation, and are therefore employed to prepare high resiliency foams. However, in the presence of conventional organotin gel catalysts even these products undergo a significant decrease in structural strength and become brittle following prolonged exposure to oxygen and/or heat.
Organotin compounds are typically very active catalysts for the reaction of isocyanates with polyols. Often the reaction is so rapid that the liquid formulation employed to prepare the foam solidifies before it can flow into every part of the mold or other container into which it is poured or before the gas generated by the blowing agent is able to escape. The volume of the entrapped gas contracts as the foam cools, resulting in shrinkage. In either instance, the dimensions of the final foam product do not coincide with those of the mold in which it is formed, and the product is rejected. The range of useful catalyst concentration for conventional foam is relatively narrow, usually between 0.25 and 0.35 part of catalyst per 100 parts of polyol. The range for high resiliency foams is slightly broader, usually between 0.01 and 0.05 part.
An objective of this invention is to improve the resistance to oxidative degradation of polyurethane foams prepared using tetravalent organotin compounds as the gel catalyst. A second objective is to reduce shrinkage and extend the useful range of gel catalyst concentration, thereby permitting a wider variation in processing conditions. Surprisingly it has now been found that if the organotin compound is added to the foam formulation as a complex with a tertiary amine, tertiary phosphine or tertiary phosphine oxide, the resultant foam exhibits improved oxidative stability relative to a foam prepared using the non-complexed organotin compound as the gel catalyst.